Encoder vs resolver

[zoeper]

Can anyone please explain the difference between encoders and resolvers for the controll of servo motors (AC brushless) in this case.
Can most ac servo amps accomodate both, or do you have to specifically check for the feedback sensor type supported.
Thanx in advance
Pieter

[Karl_T]

Resolvers are an obsolete technology. The unit outputs a sine wave. Because of low resolution, they are generally geared up from the servo motor.

Encoders output pulses, generally three channels. Two channels are used for rotation and one is an index pulse, once per revolution.

AFAIK, the servo amps for each type are different. You'll have to specify which to use.

[Al_The_Man]

In the days of DC servos, resolvers were used because of their resitance to noise, the down side was low resolution and were usually geared up, They have resurfaced now being used for commutation on sinusoidal AC servo systems, the resolver is fed into the drive first for commutation and then the result is electronically multiplied and a psuedo encoder signal complete with marker pulse is fed from drive to controller, this can usually be resolution selected by dip switch.
For the commutation, the resolver has two phases out, the third is artificially produced from a vector of the other two.
Al.

[zoeper]

What kind of resolution can you now get from a resolver with the new AC drives and are they at all suitable for CNC purposes?
You make those DC servo's sound soo old Al!
Pieter

[Al_The_Man]

The electronic mutiplication usually produces a typical 1000 to 4000 pulses/rev.
They are used exactly like you would use a normal quadrature encoder.
Al.

[zoeper]

WOW, i was just about to discard the technology completely!
Am i right in assuming then that because the encoder signal is generated by the amp, that as with steppers the controller would be unaware of steps missed by the motor??
Pieter

[Al_The_Man]

In many cases the resolver is just used for commutation by the drive, the encoder signal is used by the controller for positioning feedback.
There are also controller, cards like Galil accept the resolver signal directly into the drive, but every axis takes up two servo channels, but the resultant drive required is much simpler electronically.
Al.

[lwill]

WOW, i was just about to discard the technology completely!
Am i right in assuming then that because the encoder signal is generated by the amp, that as with steppers the controller would be unaware of steps missed by the motor??
Pieter

Remember that resolvers are ANALOG they will not work directly with most newer servo controllers, like Gecko. To use them you need to convert the anolog siginals to digital. Analog Devices offers some simple 1 and 2 chip solutions to do this convertion-
http://www.analog.com/en/prod/0,,760...2S1200,00.html
http://www.analog.com/en/prod/0,,760...2S90%2C00.html
I build a converter using the AD2S90/AD2S99 for and old Tree mill I am converting, it takes the output of the resolver and creates a 1000 ppr encoder signal. If you go to their site you can request up to 2 free samples of up to 3 devices for FREE! (this is what I did) You will need to know some electronics and not be afraid of surface mount chips.